Light absorption measurement apparatus and methods

Abstract

Apparatus and methods are disclosed for measuring changes in light absorption of a sample optical component. The sample is held in a sample holder in a sample chamber during irradiation by an intense light, usually pulsatile light. Molecules of a gas are introduced into the sample chamber as the sample is irradiated. The molecules can be of a material suspected to become adhered to or absorbed by the sample in a way that causes an undesired change in light absorption by the sample. Changes in light absorption by the sample are preferably measured photoacoustically. The molecules of the gas can be generated by controlled irradiation of a "source material" with a light (which can be the same as used to irradiate the sample) sufficient to generate such molecules of gas from the source material, and routing the gaseous molecules to the sample as the sample is irradiated.

What is claimed is: 
     
       1. An apparatus for measuring changes in light absorption exhibited by an optical component upon exposure of the optical component to light, the apparatus comprising: (a) a sample holder configured to hold a sample optical component as the sample is exposed to light;   (b) a sensor configured and situated to detect absorbance of light by the sample as the sample is exposed to the light;   (c) a sample-chamber housing configured to contain the sample holder holding the sample as the sample is exposed to the light, the sample-chamber housing defining a sample chamber; and   (d) a gas-introduction port for conducting a specified gas from a source into the sample chamber.   
     
     
       2. The apparatus of claim 1, further comprising a gas-inlet-control unit situated and configured so as to control an amount of the gas introduced into the sample chamber through the gas-introduction port. 
     
     
       3. The apparatus of claim 1, wherein the specified gas comprises molecules that can attach to the sample in a way that can result in a change in light absorbance by the sample. 
     
     
       4. The apparatus of claim 1, wherein the housing is further configured to contain a source material of molecules that can attach to the sample in a way that can result in a change in light absorbance by the sample. 
     
     
       5. The apparatus of claim 4, wherein the gas is a carrier gas. 
     
     
       6. The apparatus of claim 4, wherein the housing defines a sub-chamber for containing the source material. 
     
     
       7. The apparatus of claim 6, wherein the sub-chamber is separated from the sample chamber by a partition that blocks scattered light from passing from the sample chamber to the sub-chamber and from the sub-chamber to the sample chamber while allowing the molecules of the source material to pass from the sub-chamber to the sample chamber. 
     
     
       8. The apparatus of claim 7, wherein the housing defines a first window transmissive to a first light and a second window transmissive to a second light, the first window being situated so as to allow the first light to pass through the first window to the sample in the sample chamber, and the second window being situated so as to allow the second light to pass through the second window to the source material in the sub-chamber. 
     
     
       9. The apparatus of claim 1, wherein the housing comprises a window transmissive to the light, the window being situated so as to allow the light to pass from a light source external to the housing to the sample in the sample chamber. 
     
     
       10. The apparatus of claim 1, wherein the sensor comprises a photoacoustic transducer and a fast-fourier-transform processor connected to the photoacoustic transducer. 
     
     
       11. An apparatus for measuring changes in light absorption exhibited by an optical component upon exposure of the optical component to light, the apparatus comprising: (a) a sample holder configured to hold a sample optical component as the sample is exposed to pulses of light;   (b) a photoacoustic transducer configured and arranged to detect photoacoustic signals generated by the sample as the sample is exposed to pulses of the light;   (c) a sample chamber configured to contain the photoacoustic transducer and the sample holder holding the sample as the sample is exposed to the pulses of light, the sample chamber defining a space; and   (d) a gas-introduction port for conducting a specified gas from a source into the space defined by the sample chamber.   
     
     
       12. The apparatus of claim 11, wherein the gas comprises molecules that can adhere to or be absorbed by the sample. 
     
     
       13. The apparatus of claim 11, further comprising a source of the gas connected to the gas-introduction port. 
     
     
       14. The apparatus of claim 13, wherein the source of the gas is a source chamber connected via a conduit to the gas-introduction port, the source chamber being configured to contain a source material that, when irradiated by light, releases molecules of a compound that can adhere to or be absorbed by the sample in a way that can cause a change in light absorption by the sample. 
     
     
       15. The apparatus of claim 13, wherein the gas-introduction port comprises a gas-inlet-control unit situated and configured so as to control an amount of the gas introduced into the sample chamber through the gas-introduction port from the source. 
     
     
       16. The apparatus of claim 11, further comprising a gas sensor situated and configured to quantitatively determine a concentration of the specified gas in the space defined by the sample chamber. 
     
     
       17. The apparatus of claim 11, wherein the sample chamber comprises a material exhibiting a predetermined degassing of molecules into the space. 
     
     
       18. The apparatus of claim 17, wherein: the sample chamber comprises walls each having an interior-facing surface; and   the material is configured into a liner for the interior-facing surfaces.   
     
     
       19. The apparatus of claim 18, wherein the material is aluminum. 
     
     
       20. The apparatus of claim 18, wherein the liner is replaceable after each one or more uses. 
     
     
       21. The apparatus of claim 11, wherein the sample container is replaceable after each one or more uses. 
     
     
       22. An apparatus for measuring a change in light absorption exhibited by an optical component upon exposure of the optical component to light, the apparatus comprising: (a) a first light source operable to produce a first light;   (b) a source chamber configured to contain a source material as the source material is irradiated by the first light, the first light having a wavelength sufficient to cause the source material to produce, during such irradiation by the first light, molecules of an adhesion gas;   (c) a second light source operable to produce pulses of a second light;   (d) a sample holder configured to hold a sample optical component as the sample is exposed to pulses of the second light;   (e) a sensor configured and arranged to detect absorbance of the second light by the sample;   (f) a sample chamber defining a space, the sample chamber configured to contain within the space the sensor and the sample holder holding the sample as the sample is exposed to the pulses of the second light; and   (g) a conduit for routing molecules of the adhesion gas from the source chamber to the sample chamber so as to expose the sample to the molecules of the adhesion gas as the sample is being exposed to the second light.   
     
     
       23. The apparatus of claim 22, further comprising a light-intensity-adjustment optical system situated and configured to receive the second light and to adjust an intensity of the second light reaching the sample. 
     
     
       24. The apparatus of claim 23, further comprising a beamsplitter, wherein the source of the first light and the source of the second light are a pulsatile laser, the laser producing a light beam of which a first portion is reflected by the beamsplitter to become the first light and a second portion is transmitted by the beamsplitter to become the second light. 
     
     
       25. The apparatus of claim 22, further comprising a light blocker situated and configured to prevent stray light, including scattered light generated when the sample is irradiated by the second light, from irradiating the source material. 
     
     
       26. The apparatus of claim 22, wherein the sensor comprises a photoacoustic sensor in acoustic contact with the sample, the photoacoustic sensor being operable to measure an acoustic signal generated by expansion and contraction of the sample resulting from impingement on the sample of a pulse of the second light that causes an instantaneous heating a cooling of the sample. 
     
     
       27. The apparatus of claim 22, further comprising a gas analyzer configured and situated to obtain data regarding identity and concentration of the adhesion gas. 
     
     
       28. A method for measuring light absorption exhibited by an optical component sample as the optical component sample is irradiated with a light, the method comprising the steps: (a) mounting the sample in a sample holder;   (b) placing the sample and sample holder in a sealed environment that can contain molecules of a gas that can adhere to or be absorbed by a surface of the sample;   (c) irradiating the sample with pulses of a first light while exposing the sample to the molecules of the gas in the sealed environment containing molecules of the gas; and   (d) as each pulse of the first light impinges on the sample, measuring an acoustic signal generated in the sample due to exposure to the pulse, the acoustic signal being a function of an amount of light energy absorbed by the sample from the light pulse.   
     
     
       29. The method of claim 28, further comprising the step of generating an electrical signal corresponding to the acoustic signal. 
     
     
       30. The method of claim 29, further comprising the step of performing a fast fourier transform of the electrical signal. 
     
     
       31. The method of claim 28, further comprising the steps of: irradiating a source material with a second light to cause the source material to produce molecules of the gas; and   conducting the molecules of the gas to the sealed environment so as to contact the sample as the sample is being exposed to the first light in the sealed environment.   
     
     
       32. A method for measuring light absorption exhibited by an optical component sample as the sample is irradiated with a light, the method comprising the steps of: (a) providing a sealable sample chamber;   (b) placing the sample in the sample chamber and sealing the sample chamber;   (c) providing molecules of an adhesion gas;   (d) while introducing the molecules of the adhesion gas into the sample chamber, irradiating the sample in the sample chamber with pulses of a light suspected of causing the sample to exhibit a change in absorption of the first light with cumulative exposure of the sample to pulses of the light; and   (e) measuring the absorption by the sample of the pulses of the light.   
     
     
       33. The method of claim 32, wherein step (a) comprises providing a sample chamber defined by walls exhibiting low degassing. 
     
     
       34. The method of claim 32, wherein step (a) comprises providing a sample chamber defined by walls lined with a material exhibiting low degassing. 
     
     
       35. A method for measuring light absorption exhibited by an optical component sample as the sample is irradiated with a light, the method comprising the steps of: (a) irradiating the sample with a pulse of a light suspected of causing the sample to exhibit a change in absorption of the light with cumulative exposure of the sample to pulses of the light;   (b) as the pulse of the light impinges on the sample, measuring an acoustic signal generated in the sample due to exposure to the pulse, the acoustic signal having a waveform that is a function of an amount of light energy absorbed by the sample from the light pulse and being generated by an expansion and contraction of the sample due to an instantaneous heating of the sample as the sample receives and absorbs at least a portion of the light pulse and a subsequent cooling of the sample after the light pulse; and   (c) determining from the waveform, generated as the sample received the pulse of the light, a first waveform component arising from absorption of light from the pulse on a surface of the sample and a second waveform component arising from absorption of light from the pulse within a depth dimension of the sample.   
     
     
       36. The method of claim 35, wherein step (c) further comprises: separating the first waveform component from the second waveform component;   breaking down the first and second waveform components into respective constituent frequency components; and   comparing the amplitude of the frequency components of the first waveform with the amplitude of the frequency components of the second waveform.   
     
     
       37. The method of claim 35, further comprising the step of comparing information obtained in step (c) with corresponding information obtained in step (c) from an earlier pulse irradiated on the sample. 
     
     
       38. The method of claim 35, further comprising the step of comparing information obtained in step (c) with corresponding information previously obtained in step (c) from irradiating a different sample with a pulse of the light.